A lipoma is a benign tumor composed of adipose tissue (body fat). It is the most common benign form of soft tissue tumor.[1] Lipomas are soft to the touch, usually movable, and are generally painless. Many lipomas are small (under one centimeter diameter) but can enlarge to sizes greater than six centimeters. Lipomas are commonly found in adults from 40 to 60 years of age, but can also be found in younger adults and children. Some sources claim that malignant transformation can occur,[2] while others say this has yet to be convincingly documented.[3]

Chondroid lipomas are deep-seated, firm, yellow tumors that characteristically occur on the legs of women.[4]:625

Corpus callosum lipoma is a rare congenital brain condition that may or may not present with symptoms.[8] This occurs in the corpus callosum, also known as the calossal commissure, which is a wide, flat bundle of neural fibers beneath the cortex in the human brain.

Intradermal spindle cell lipomas are distinct in that they most commonly affect women and have a wide distribution, occurring with relatively equal frequency on the head and neck, trunk, and upper and lower extremities.[4]:625[6]

Neural fibrolipomas are overgrowths of fibro-fatty tissue along a nerve trunk, which often leads to nerve compression.[4]:625

Pleomorphic lipomas, like spindle-cell lipomas, occur for the most part on the backs and necks of elderly men and are characterized by floret giant cells with overlapping nuclei.[4]:625

Spindle-cell lipomas are asymptomatic, slow-growing subcutaneous tumors that have a predilection for the posterior back, neck, and shoulders of older men.[4]:625

Superficial subcutaneous lipomas, the most common type of lipoma, lie just below the surface of the skin.[3] Most occur on the trunk, thigh, and forearm, although they may be found anywhere in the body where fat is located.

The tendency to develop a lipoma is not necessarily hereditary although hereditary conditions, such as familial multiple lipomatosis, may include lipoma development.[9][10] Genetic studies in mice have shown a correlation between the HMG I-C gene (previously identified as a gene related to obesity) and lipoma development. These studies support prior epidemiologic data in humans showing a correlation between HMG I-C and mesenchymal tumors.[11]

Cases have been reported where minor injuries are alleged to have triggered the growth of a lipoma, called "post-traumatic lipomas."[12] However, the link between trauma and the development of lipomas is controversial.[13]

Lipomatosis is believed to be a hereditary condition in which multiple lipomas are present on the body.

Adiposis dolorosa (Dercum disease) is a rare condition involving multiple painful lipomas, swelling, and fatigue. Early studies mentioned prevalence in obese postmenopausal women. However, current literature demonstrates that Dercum disease is present in more women than men of all body types; the average age for diagnosis is 35 years.[14]

Usually, treatment of a lipoma is not necessary, unless the tumor becomes painful or restricts movement. They are usually removed for cosmetic reasons, if they grow very large, or for histopathology to check that they are not a more dangerous type of tumor such as a liposarcoma.[3] This last point can be important as the characteristics of a "bump" are not known until after it is removed and medically examined.

Lipomas are normally removed by simple excision.[15] The removal can often be done under local anaesthetic, and takes less than 30 minutes. This cures the great majority of cases, with about 1–2% of lipomas recurring after excision.[16]Liposuction is another option if the lipoma is soft and has a small connective tissue component. Liposuction typically results in less scarring; however, with large lipomas it may fail to remove the entire tumor, which can lead to regrowth.[17]

Lipomas are rarely life-threatening and the common subcutaneous lipomas are not a serious condition. Lipomas growing in internal organs can be more dangerous; for example lipomas in the gastrointestinal tract can cause bleeding, ulceration and painful obstructions (so-called "malignant by location", despite being a benign growth histologically).[19][20] Malignant transformation of lipomas into liposarcomas is very rare and most liposarcomas are not produced from pre-existing benign lesions.[16] A few cases of malignant transformation have been described for bone and kidney lipomas,[21][22] but it is possible these few reported cases were well-differentiated liposarcomas in which the subtle malignant characteristics were missed when the tumour was first examined.[23] Deep lipomas have a greater tendency to recur than superficial lipomas, because complete surgical removal of deep lipomas is not always possible.[23][24]

The presence of multiple lipomas is known as lipomatosis and is more commonly encountered in men. Some superficial lipomas can extend into deep fascia and may complicate excision. Liposarcoma is found in 1% of lipomas and are more likely in lesions of the lower extremities, shoulders and retroperitoneal areas. Other risk factors for liposarcoma include large size (>5 cm), associated with calcification, rapid growth and/or invasion into nearby structures or through fascia into muscle tissue.[25]

Approximately one percent of the general population has a lipoma.[3] These tumors can occur at any age, but are most common in middle age, often appearing in people from 40 to 60 years old.[15] Cutaneous lipomas are rare in children, but these tumors can occur as part of the inherited disease Bannayan-Zonana syndrome.[26][27]

Lipomas are usually relatively small with diameters of about 1–3 cm,[28] but in rare cases they can grow over several years into "giant lipomas" that are 10–20 cm across and weigh up to 4–5 kg.[29][30]

Lipomas occur in many animals, but are most common in older dogs, particularly older Labrador Retrievers, Doberman Pinschers and Miniature Schnauzers.[31] Overweight female dogs are especially prone to developing these tumors and most older or overweight dogs have at least one lipoma.[32][33] In dogs, lipomas usually occur in the trunk or upper limbs.[31] Lipomas are also found less commonly in cattle and horses, and rarely in cats and pigs.[33][34] However, a pedunculated lipoma can cause entrapment and torsion of the intestine in horses, causing necrosis, colic, and possibly death. The intestine becomes wound around the stalk of the lipoma and loses blood supply.[35]